Storable propellants for nuclear engine?

[xenomorph555]

Okay with regards to nuclear engines we know liquid hydrogen is the best propellant. You can launch, get to Mars in a month and enter orbit. But when you wan't to return in a month you would need a second nuclear stage, but as hydrogen isn't that storable we would need something else. Any suggestions?

[Starman4308]

Ammonia and methane are both reasonable choices. Methane would probably require a bit of active cooling, and has issues with leaving carbon residue. Ammonia has one amusing problem: there's a good chance you might need to melt it before use.

[xenomorph555]

Ammonia and methane are both reasonable choices. Methane would probably require a bit of active cooling, and has issues with leaving carbon residue. Ammonia has one amusing problem: there's a good chance you might need to melt it before use.

Hmm very interesting, I do have an idea to possibly overcome the ammonia problem:

T-tank wall

R-reactor

A-ammonia

N-nozzle

S-space

SSSTTTTSSS

SSTARATSS

STAARAATS

TAAARAAAT

STTTRTTTTS

SSSSNSSSS

(It keeps sliding to the side for some reason D:<)

The stage would be similar to a pyramid/cone with a long reactor running down the center spine of it and a nozzle at the bottom. The thing would solid stage prior to activation in Mars or Venus orbit, at first the engine nozzle would be closed and the reactor slowly started up to a heating temperature where the ammonia surrounding the spine would begin to melt and create pressure the free floating molecules would allow temperature to flow through the pyramid causing most ammonia to become liquid and super high pressure. At this point the reactor would be ordered to go to full temperature and the engine nozzle opened to allow the high temperature, energy, pressure ammonia to escape and create the thrust.

Edited December 18, 2014 by xenomorph555

[Starman4308]

Hmm very interesting, I do have an idea to possibly overcome the ammonia problem:

That sounds excessively complex, particularly at the "very high pressure" part. Also, I'm not sure you can get NTR reactors to go from partial to full output on a dime like that.

How about you just run some of the reactor coolant through lines running through the fuel tank? It wouldn't require much piping, and it might be a good idea to keep the propellant liquid anyways, so as to avoid any freeze/thaw issues from cropping up.

[xenomorph555]

That sounds excessively complex, particularly at the "very high pressure" part. Also, I'm not sure you can get NTR reactors to go from partial to full output on a dime like that.

How about you just run some of the reactor coolant through lines running through the fuel tank? It wouldn't require much piping, and it might be a good idea to keep the propellant liquid anyways, so as to avoid any freeze/thaw issues from cropping up.

Yes a much better idea, if only I could transfer these drawings Iv'e been making to the thread to show what it would look like. Anyway I'm off to China to propose the idea LOLOLOL, not really...

[cryogen]

You're talking about nuclear thermal rockets specifically. Actually, there's no point in using any propellant other than LH2 -- that's the only reason they have high Isp. Small molecules move faster than heavy ones -- the speed scales as 1/sqrt(molecule mass) according to equipartition. This translates to exhaust velocity (Isp). NTR's propellant is H2, with a mass of 2 amu: that gives it its edge over chemical LH2/LOX rockets, whose propellant stream is mainly H2O with mass 18 amu.

NTR's don't run hotter than chemical rockets, so with a heavier propellant (like CH4), the Isp advantage probably wouldn't be there. Barring exotic ideas like gas-core reactors, they're limited by the temperature of the solid reactor fuel, that's acting as a heat exchanger.

It's actually a soluble problem though (LH2 boiloff): you can carry along a refrigeration system to keep the LH2 below boiling. In NASA's bimodal NTR concept, it'd cost about 1 ton of mass and 10 kWe power. If you're building large enough rockets, LH2 is a storable propellant.

(There's already precedent for this: Hubble has its own cryogenic cooler. It has this ridiculously tiny gas turbine that keeps liquid neon below 70 K, to keep thermal noise out of the infrared camera).

[Nuke]

it would be more versatile to have an engine that can switch propellants during the mission. hydrogen gives you the best isp, but if you cant find it for some reason, you can still put something in the tank and get some deltav out of it. of course this really only matters if you have isru capability. if its a one shot mission with no refueling just tank up with hydrogen.

[AngelLestat]

For nuclear thermal many things would work..

just water for example..

[Fuzzy Dunlop]

If your not going to use Hydrogen then there's really no point in Nuclear Thermal Rocketry. The ISP of an engine is proportional to the exhaust velocity, which is affected by two factors: Tempurature and the Molar Mass of the gas. NTR engines don't run at a higher tempurature than chemical rockets but they can use a (much) lighter exhaust product, pure Hydrogen. This is where all the ISP gain comes from and if you use a much heavier gas like methane or ammonia thats no more efficient than a Hydrolox rocket.

[AngelLestat]

You have more ISP with hydrogen.. but how much hydrogen mass you can have in a fixed tank volume?

If I am in space aside an asteroid, I would have more deltaV if I full the tank with water than with hydrogen, (that is my opinion, I never thought much about it)

Of course if you launch the rocket from earth surface, and your second or third stage is your nuclear thermal rocket, then is always better hydrogen. The same than any chemical engine.

[cryogen]

I found out that Zubrin looked at NTR propellants other than hydrogen. His idea (1991) was to land an NTR on the surface of Mars, and use the NTR as an ascent rocket using Martian fuel (CO2). (NASA's modern concept is to bring a refrigerated zero-boil-off LH2 tank to Mars, and leave that in Mars orbit for the return trip. The surface ascent would be CH4/LOX, the CH4 from Martian ISRU).

Some of the theoretical Isp's Zubrin cites are (at 2800 K): 283 s for CO2, 370 s for H2O, and 606 s for CH4. (So in particular, CH4 can be much better than chemical LH2/LOX at 450 s, but still worse than hydrogen NTR at ~900 s).

Nuclear Rocket Using Indigenous Martian Fuel (Zubrin, 1991)

Mars Design Reference Architecture 5.0 (NASA, 2009)

Edited January 10, 2015 by cryogen

[Nuke]

you dont need as much fuel for the return trip, especially if you use aerocapture (mars <-> earth run for example can aerocaptue at both places). so low isp fuel works there. i also imagine doing a moon <-> mars run with isru fuel produced at both ends being a thing. the nuclear reactor would be usable for many trips before it needed to have its core refueled. i also have the feeling we wont routinely run nuclear ships (for obvious political reasons) till we have an off world source of the nuclear fuel somewhere.

[KerikBalm]

http://www.projectrho.com/public_html/rocket/enginelist.php#ntrsolidcore

Solid Core NTR

3200° K

Exhaust velocity (H2) 8,093 m/s

Exhaust velocity (CH4) 6,318 m/s

Exhaust velocity (NH3) 5,101 m/s

Exhaust velocity (H2O) 4,042 m/s

Exhaust velocity (CO2) 3,306 m/s

Exhaust velocity (CO or N2) 2,649 m/s

I'm just going to divide by 9.81 to get Isps...

LH2 gets 825

Methane gets 644

Ammonia gets 519

All of those are better than LH2/LOx chemical rockets that get 451

Also, you can get better TWRs with those lower Isp propellants.

If we had a reactor core at 3500, instead of 3200, then NH3 would get about 545 Isp

We should also consider that if LH2 won't be available for the return trip for nuclear, then it won't be for chemical either, and these nuclear thermal propellants would be competing against the likes of propane

Pretty good, all things considered.

Edited January 10, 2015 by KerikBalm

[NERVAfan]

http://www.projectrho.com/public_html/rocket/enginelist.php#ntrsolidcore

We should also consider that if LH2 won't be available for the return trip for nuclear, then it won't be for chemical either, and these nuclear thermal propellants would be competing against the likes of propane

Yeah... IIRC SpaceX plans for their methane/LOX Mars engine (Raptor) to have 380 specific impulse in vacuum, which is good, but still a lot less than 450-460 for LH2/LOX.

[Fuzzy Dunlop]

If we had a reactor core at 3500, instead of 3200, then NH3 would get about 545 Isp

We should also consider that if LH2 won't be available for the return trip for nuclear, then it won't be for chemical either, and these nuclear thermal propellants would be competing against the likes of propane

Pretty good, all things considered.

Yeah but Uranium Dioxide melts at 3138K. The NERVA prototypes heated their propellent to at most 2550K.

[KerikBalm]

Still, methane gets you much better Isp even at that temperature.

Of course, we can imagine gas core designs and such getting those temperatures.

Particularly appealing is the use of CO2 and ISRU for Mars. 310 Isp is nothing to sneeze at if you can pick your propellent up at mars.

[AngelLestat]

I find Nerva engines only usefull as an interplanetary cargo ship always working close to 0g with fuel rechargeable.

So you can stop in the moon or any asteroid to fill your tank (I guess water is the best) and keep going.

[NERVAfan]

Exhaust velocity (CH4) 6,318 m/s

Exhaust velocity (NH3) 5,101 m/s

Exhaust velocity (H2O) 4,042 m/s

Can that be right? Molecular weight CH4 = 16, NH3 = 17, H2O = 18. Why is the difference between them so big?

[Kryten]

Can that be right? Molecular weight CH4 = 16, NH3 = 17, H2O = 18. Why is the difference between them so big?

The temperature in the reactor is going to be high enough to cause partial decomposition, the exhaust won't just be pure fuel.

[NERVAfan]

The temperature in the reactor is going to be high enough to cause partial decomposition, the exhaust won't just be pure fuel.

Ok, but even then, I still don't understand why methane is so good. I don't see what it can decompose to except elemental carbon and hydrogen gas... but elemental carbon shouldn't be a gas at 3200 K (which is what those numbers are for). Looking for phase diagrams it seems to be at least 4000 K for gaseous carbon even at zero pressure.

So shouldn't the carbon be falling out of the exhaust as a solid and actually making your specific impulse worse by effectively losing 3/4 of the mass you put in (or ejecting it at very low velocity)?

Whereas ammonia should decompose to two gases.

[Kryten]

Most of the carbon should convert to acetylene or ethylene, not elemental carbon.

[FreeThinker]

I wonder if Diborane would work as a nuclear propellant. In theary it should be great as it decomposes into 5 molecules which on every has a atomic mass of only 3.2, which would mean the Isp should be 79% of Hydrogen but much more compact at 0.421 kg /l

Edited April 5, 2016 by FreeThinker

[justidutch]

If one were to use methane, I wonder how many cows would be sufficient to keep a steady supply going?

 

Sorry about that.  Back to your regular programming...

[fredinno]

On 12/16/2014 at 3:45 PM, xenomorph555 said:

Okay with regards to nuclear engines we know liquid hydrogen is the best propellant. You can launch, get to Mars in a month and enter orbit. But when you wan't to return in a month you would need a second nuclear stage, but as hydrogen isn't that storable we would need something else. Any suggestions?

If you want efficiency, Lithium might be a good idea. It's very light (~7g/mol vs CH4's ~16g/mol and H2's 2g/mol), but needs pre-heating to get it moving. It is also reactive, so .

Edited April 5, 2016 by fredinno

[fredinno]

On 1/8/2015 at 1:13 AM, cryogen said:

You're talking about nuclear thermal rockets specifically. Actually, there's no point in using any propellant other than LH2 -- that's the only reason they have high Isp. Small molecules move faster than heavy ones -- the speed scales as 1/sqrt(molecule mass) according to equipartition. This translates to exhaust velocity (Isp). NTR's propellant is H2, with a mass of 2 amu: that gives it its edge over chemical LH2/LOX rockets, whose propellant stream is mainly H2O with mass 18 amu.

NTR's don't run hotter than chemical rockets, so with a heavier propellant (like CH4), the Isp advantage probably wouldn't be there. Barring exotic ideas like gas-core reactors, they're limited by the temperature of the solid reactor fuel, that's acting as a heat exchanger.

It's actually a soluble problem though (LH2 boiloff): you can carry along a refrigeration system to keep the LH2 below boiling. In NASA's bimodal NTR concept, it'd cost about 1 ton of mass and 10 kWe power. If you're building large enough rockets, LH2 is a storable propellant.

(There's already precedent for this: Hubble has its own cryogenic cooler. It has this ridiculously tiny gas turbine that keeps liquid neon below 70 K, to keep thermal noise out of the infrared camera).

But those coolers are expensive. Would a Sun-shield cost less?

On 1/8/2015 at 2:58 PM, AngelLestat said:

You have more ISP with hydrogen.. but how much hydrogen mass you can have in a fixed tank volume?

If I am in space aside an asteroid, I would have more deltaV if I full the tank with water than with hydrogen, (that is my opinion, I never thought much about it)

Of course if you launch the rocket from earth surface, and your second or third stage is your nuclear thermal rocket, then is always better hydrogen. The same than any chemical engine.

Tank volume isn't very important in space.

On 1/9/2015 at 3:02 AM, Nuke said:

you dont need as much fuel for the return trip, especially if you use aerocapture (mars <-> earth run for example can aerocaptue at both places). so low isp fuel works there. i also imagine doing a moon <-> mars run with isru fuel produced at both ends being a thing. the nuclear reactor would be usable for many trips before it needed to have its core refueled. i also have the feeling we wont routinely run nuclear ships (for obvious political reasons) till we have an off world source of the nuclear fuel somewhere.

The Moon is bad for ISRU, since it lacks H2 (except for the precious stuff at the poles). You would need to bring your H2 (though that is light). But even then, the moon also lacks N2 and C, so you'd be forced to use water as fuel for an NTR instead. And those suck in ISP. 

On 1/9/2015 at 0:24 PM, NERVAfan said:

Yeah... IIRC SpaceX plans for their methane/LOX Mars engine (Raptor) to have 380 specific impulse in vacuum, which is good, but still a lot less than 450-460 for LH2/LOX.

But it may be better accounting for boil-off.

On 1/9/2015 at 0:55 PM, Fuzzy Dunlop said:

Yeah but Uranium Dioxide melts at 3138K. The NERVA prototypes heated their propellent to at most 2550K.

The limitation is not reactor material temperature, but the temperatures of the lining.

On 1/11/2015 at 9:05 PM, Kryten said:

Most of the carbon should convert to acetylene or ethylene, not elemental carbon.

The carbon still looks like it would be bad to use CH4 in a reusable nuclear tug.

1 hour ago, FreeThinker said:

I wonder if Diborane would work as a nuclear propellant. In theary it should be great as it decomposes into 8 molecules which on every has a atomic mass of only 2, which would mean the Isp should only be slight lower than Hydrogen but much more compact

No, Diborane has a much higher molar mass than CH4, so it's actually way worse.

1 hour ago, justidutch said:

If one were to use methane, I wonder how many cows would be sufficient to keep a steady supply going?

 

Sorry about that.  Back to your regular programming...

A lot. You'd be better off burning the hay to feed them as fuel.